Oriented polyester film, particularly biaxially oriented film composed of polyethylene terephthalate (PET) is widely used as a packaging and label material for a variety of products, including foodstuffs.
Because the surface of oriented PET film is hydrophobic, it is not readily receptive to coating or printing. In many packaging and label applications it is advantageous that the film be receptive to ordinary flexographic and rotogravure printing inks.
In an attempt to remedy this deficiency, special inks have been devised that adhere directly to the surface of PET film. However, many such inks are based on formulations of organic solvents, resinous binders and other ingredients which are costly, difficult to handle and hazardous for use in the manufacturing plant.
Another approach for improving the printability of PET film is to apply a primer coating which adheres to the film and is receptive as well to printing inks applied thereto. Examples of such primer coatings include polymeric compositions based on vinylidene chloride terpolymers such as disclosed in U.S. Pat. No. 2,698,240 or water dispersible copolyester primers such as disclosed in U.S. Pat. No. 4,252,885. The problem with this approach to enhancing ink adhesion to PET film is that the film is in many instances used as a food packaging material and consequently the relative toxicity of the primer layer becomes important. Though polyester film per se is approved as a food packaging material by most Governmental agencies, including the U.S. Food and Drug Administration, polyester film primed with certain polymeric primer coatings such as acrylics, vinylidene chloride-containing polymers and many other similar primers raised toxicity problems because of potential migration of said primer material into foodstuffs with which it is in contact.
It is known to include small quantities of sodium alkyl sulfates into aqueous dispersions of resinous materials which dispersions are employed as a primer layer for biaxially oriented PET film. The sulfates serve as emulsifiers or wetting agents in such applications, such as taught in U.S. Pat. No. 2,627,088. It is known to utilize various organic or inorganic salts as slip agents in the production of polyolefin films such as disclosed in U.S. Pat. No. 3,277,227, and it is also known to coat polypropylene film with certain fatty acid amides to impart anti-blocking and lubricity characteristics as taught in U.S. Pat. No. 4,225,644. British Pat. No. 1,000,019 teaches a method for improving the slip properties of biaxially oriented PET film by coating the film with an aqueous solution comprising sodium silicate, amorphous silica and a surface active agent. U.S. Pat. No. 3,715,421 suggests the use of certain fatty acid salts as lubricating finishes for polyester filaments. U.S. Pat. No. 3,393,081 teaches the application of a thin film of a saturated fatty acid and/or a fatty alcohol to biaxially oriented polystyrene sheet material in order to impart the property of heat sealability to the film.
None of these prior art disclosures, however, are addressed to the problem of the enhancement of the printability of polyester film and the solution of that problem by the technique hereinafter disclosed.
Accordingly it is an object of this invention to provide an oriented, self-supporting polyester film material having enhanced receptivity to flexographic or rotogravure printing inks.
Another object is to provide a primer coating for oriented polyester film material which can safely and easily be applied in the plant during production of the film.
A further object is to provide a primer material for biaxially oriented polyethylene terephthalate film material which renders such film generally receptive to flexographic or rotogravure printing inks for packaging applications.